This invention relates to a correction device for correcting upper and lower pincushion distortions in pictures displayed on a color picture tube provided with an electromagnetic deflection unit, and more particularly to an upper and lower pincushion distortion correction device suitable for a high speed horizontal deflection at a frequency of, for example, 130 KHz.
In a display apparatus such as a color picture tube using a deflection yoke unit adapted for generation of a magnetic field of a substantially uniform field distribution, there occur, in rasters at upper and lower portions of a screen, picture distortions in the form of a pincushion (hereinafter referred to as upper and lower pincushion distortions). Accordingly, it has hitherto been proposed to superimpose a correction current for correction of upper and lower pincushion distortions upon a vertical deflection current flowing through vertical deflection coils, as disclosed in U.S. Pat. No. 3,760,222. However, in high-definition display using a high frequency (about 130 KHz) for horizontal deflection, a horizontal component leaks into a vertical component with the result that a ringing current is generated in the vertical deflection current owing to the horizontal component or its higher harmonics.
Japanese Patent Publication No. 52-32927 discloses an expedient wherein an auxiliary deflection coil is disposed behind a main deflection coil assembly, and a current for correcting upper and lower pincushion distortions is supplied to the auxiliary deflection coil. This expedient has succeeded in preventing the ringing current in the auxiliary deflection coil but raises a problem that the current flowing through the auxiliary deflection coil will change the deflection center to thereby change conditions for electron beam landing on the fluorescent screen.